This invention relates generally to pressure vessels having adaptor fittings mounted thereon to provide access to the interior of the vessel. The invention is particularly concerned with the techniques for manufacturing filament- wound plastic pressure vessels which contain corrosive and/or pressurized fluids. Pressure vessels of this type generally include a rotationally cast or blow-molded inner plastic liner which is wound with resin-impregnated glass filaments to provide adequate mechanical and structural properties for the sidewall of the vessel. A typical pressure vessel would comprise a cylindrical sidewall having oblate, ellipsoidal end portions, at least one of which is provided with an axial fitting which, in turn, is adapted to be connected to a closure member or to plumbing connections. According to the prior art, metallic and plastic fittings are associated with plastic liners by fusion or molding techniques, and the sub-assembly of the fitting and the liner are wound with resin-impregnated filaments. Examples of such pressure vessels may be found in U.S. Pat. Nos. 2848133; 3874544; 3508677; 3907149; and 4705468.
In U.S. Pat. No. 4,705,468 granted to LeBreton, there is disclosed a technique for rotationally casting a plastic liner for a pressure vessel having an access fitting molded therein. According to that patent, the apparatus includes a rotational casting arm having a mold supporting surface which is adapted to be rotated about orthogonal axes. A hollow mold having a mold cavity surface conforming to the outside surface of the liner to be molded is removably attached to the mold supporting surface together with a metal access fitting. The metal access fitting has a longitudinal axis aligned with one of the orthogonal axes and has at least a first surface portion extending into the mold cavity. The first surface portion and the surface of the mold cavity are coated with plastic upon molding rotation to form the plastic liner.
It has been found that in practicing the invention set forth in the LeBreton patent, the plastic resin which coats the first surface portion of the fitting tends to be thinner than the remainder of the tank liner since the relatively large mass of the fitting does not attain the same elevated temperature as the relatively thin mold wall sections during the dwell time of the mold in the heating chamber. As a consequence of this molding operation, a relatively thin coating of resin is formed on the interior surface of the fitting which may permit liquid contained in a completed pressure vessel to seep through and contact the fitting itself. Moreover and in most cases, a thin plastic lining on the interior of the fitting tends to delaminate and pull away from the fitting upon cooling. To overcome this tendency, excess molding composition is used to ensure that the coating on the interior of the fitting is of sufficient thickness. While in many cases excess molding composition is not detrimental in an engineering sense, it necessarily adds to the cost and weight of the item. When expensive molding compounds are employed, significant cost increases in the finished product will result.